The present invention refers to a valve assembly for venting die casting moulds, comprising a venting channel, a venting valve communicating with the venting channel and having an axially movable valve closure member, and an actuating device for operating the venting valve from an open position to a closed position.
In order to reliably avoid the occurrence of air inclusions in the finished casting during the casting operation, the mould and the cavity in the mould, respectively, have to be vented during the casting operation. Thereby, not only the air contained in the cavity of the mould has to be allowed to escape, but in addition it must be ensured that also the gases escaping from the liquid casting material are removed from the mould cavity.
One of the problems in connection with venting die casting moulds can be seen in the requirement that the venting valve of the valve assembly be closed as late as possible in order to ensure that the mould cavity is vented until it is fully filled with liquid casting material, but that it is also to be avoided that liquid casting material enters the venting valve.
In order to take this problem into account, generally two kinds of valve assemblies for die casting moulds are known, whereby in either case a venting valve is provided that is equipped with a axially back and forth movable valve piston for closing the venting channel. While the valve piston is moved by suitable driving means in a first kind of valve assemblies, the valve piston of a second kind of valve assemblies is operatively connected to a power pick-up member that is operated directly by the liquid casting material flowing from the cavity of the mould into the venting channel.
Suitable driving means for the above mentioned first kind of valve assemblies may include pneumatically or hydraulically operated driving systems for moving the valve piston. The moment in which the closing of the venting valve is initiated can be determined, for example, by means of a sensor that monitors the level of the mould cavity. However, one difficulty observed with such systems consists in the fact that the closing operation takes a considerably long time because the signal initiating the closing operation, mostly an electric signal, has to be transformed into a mechanical movement, for example into the operation of a servo valve. Moreover, for the purpose of closing the venting valve or for the purpose of operating an actuating member that is operatively connected to the valve piston of the venting valve, a predetermined system pressure must be available in order to ensure that the venting valve can be pneumatically or hydraulically closed within the required time period. However, since the operation of a servo valve usually causes a drop in system pressure, it is necessary to rebuild the system pressure again before the servo valve can be closed. It is understood that such valve assemblies are of a quite complicated design and require a high expenditure; moreover, they are subject to be influenced by certain operation parameters.
In contrary, with the second kind of valve assemblies, it is possible to realize very quick acting and reliable venting devices. In order to ensure that a ram pressure can be built up that is high enough to operate the venting valve piston, the venting channel leading from the mould cavity to the power pick-up member is provided with a number of deviations and constrictions. Moreover, the venting channel must have a certain minimal distance and has to be of angled design between the power pick-up member and the real valve body member of the venting valve, in order to ensure that the venting valve is safely closed before the liquid casting material has reached the venting valve. In order to increase the efficiency of such valve assemblies, usually a vacuum pump is connected to the venting valve.
U.S. Pat. No. 5,488,985 discloses a valve assembly for venting die casting moulds of the second kind, as described herein above, that comprises a venting channel, a venting valve communicating with the venting channel and an actuating device for closing the venting valve. The actuating device includes a power pick-up member that is operated by the liquid casting material flowing from the mould cavity into the venting channel. The movable closure member of the venting valve is mechanically operatively coupled to the power pick-up member. The power-pick-up member is designed as a push member whose operating stroke is limited to a fraction of the length of the closing stroke of the closure member of the venting valve. Moreover, the closure member of the venting valve is freely movable beyond the operating stroke of the push member. The actuating device comprises a power transmission member for transmitting the impact force from the power pick-up member to the movable closure member of the venting valve.
Even if such a valve assembly operates very reliably in practice, it would be desirable in certain circumstances if the energy required for closing the venting valve would not be raised by the flowing liquid casting material alone. As it is evident from the general formula for calculating the kinetic energy (E=mxc2x7v2/2), the energy available for closing the venting valve depends on the mass and the velocity of the liquid casting material. In other words, under certain unfavorable circumstances, the available energy may be not sufficient for closing the venting valve within the required time period, particularly in the case of a low mass of casting material and/or in the case of slowly flowing casting material.
Thus, it is an object of the invention to provide a valve assembly for venting die casting moulds that always operates reliably and safely, even under the worst operating conditions.
In order to meet this and other objects, the present invention provides, according to a first aspect, a valve assembly for venting die casting moulds, comprising a venting channel, a venting valve communicating with the venting channel and having an axially movable valve closure member, and an actuating device for operating the venting valve from an open position to a closed position.
An axially movable stop member is adapted to be axially displaced under the influence of the liquid casting material entering the venting channel. The actuating device thereby includes an axially movable actuating element that is operatively connected to the valve closure member and biased towards the stop member such that the valve closure member of the venting valve is movable from its open position to its closed position under the effect of the actuating element.
According to a second aspect, the present invention also provides a valve assembly for venting die casting moulds, comprising a venting channel, a venting valve communicating with the venting channel and having an axially movable valve closure member, and an actuating device for operating the venting valve from an open position to a closed position.
Further, there are provided an axially movable stop member and means for axially displacing the stop member.
The actuating device includes an axially movable actuating element operatively connected to the valve closure member and biased towards the stop member such that the valve closure member of the venting valve is movable from its open position to its closed position under the effect of the actuating element.
Due to the fact that the actuating device of the valve assembly comprises an actuating element that is biased against a stop member and operatively coupled to the closure member of the venting valve, and further due to the fact that the stop member can be operated either by separate driving means or by the casting material itself such that the closure member of the venting valve is movable from its open position to its closed position under the effect of the actuating element driven by the stop member, it is not necessary first to raise a certain force (for example pneumatically) before the closure element of the venting valve can be moved from its open to its closed position once the actuating element is released, i.e. the stop member has moved. This is important insofar as in this way, on the one hand, the closing time of the venting valve can be minimized and, on the other hand, only a short and relatively small impact force acting on the stop member is sufficient for safely and reliably closing the venting valve. As already mentioned, the stop member can be moved under the influence of separate driving means, or under the influence of the casting material. Such separate driving means may include pneumatic, hydraulic or electromagnetic drives. In the case of moving the stop member by the casting material itself, it is understood that the kinetic energy of the flowing liquid casting material is used for moving the stop member. Thereby, both a direct and an indirect operation, e.g. by means of a power pick-up member can be realized.